1. Field of the Invention
The present invention relates to a shadow mask for a color CRT (cathode ray tube), and in particular to a slot interval of a shadow mask for a color CRT (cathode ray tube).
2. Description of the Prior Art
FIG. 1 is a schematic view illustrating a general color CRT (cathode ray tube), and FIG. 2 is a perspective view illustrating a general shadow mask.
Generally, as depicted in FIG. 1, in a color CRT, a panel 10 as a front glass is combined with a funnel 20 as a rear glass, and they are hermetically sealed as high vacuum.
And, the color CRT includes a fluorescent surface 40 coated to the inner surface of the panel 10 and performing a luminescent material function, an electron gun 130 discharging an electron beam 60 for radiating the fluorescent surface 40, a shadow mask 70 for making the electron beam from the electron gun 130 land on a certain portion of the fluorescent surface 40, a frame 30 fixing/supporting the shadow mask 70, a spring 80 and a stud pin 120 for combining the frame assembly 30 with the panel 10, and an inner shield 90 combined with a certain surface of the frame 30 and extended from the panel side to the funnel side in order to protect the CRT from influences of terrestrial magnetism in operation.
And, the electron gun 130 for generating the electron beam 60 is installed to the internal surface of the neck portion of the funnel 20, a deflection yoke 50 is installed to the outer surface of the neck portion of the funnel 20 in order to deflect the electron beam 60 from the electron gun 130 in a certain direction, and a CPM (convergence and purity magnet) 100 for precisely adjusting the deflected direction of the electron beam 60.
And, a reinforcing band 110 is installed to the outer circumference of the combining portion of the panel 10 and the funnel 20 in order to help the panel 10 and the funnel 20 stand an air pressure and external impacts.
In addition, as depicted in FIG. 2, the conventional shadow mask 70 includes an effective surface 74 having a plurality of circular-shaped or oval-shaped slots 72 and a skirt 76 having a certain length so as to be welded to the frame 30.
FIG. 3 illustrates a part of the effective surface 74 of the shadow mask 70, in order to make the electron beam 60 maintain a certain interval when it passes the slots 72 on the effective surface 74 of the shadow mask 70 and lands on the fluorescent surface 40 coated on the panel 10, the slots 72 respectively having a certain sized hole are arranged horizontally and vertically at regular intervals.
In addition, on the shadow mask 70, in order to maintain a strength after fabrication, each bridge 78 is formed between abutted two vertical slots 72.
In FIGS. 3 and 4, a non-explained reference numeral Ph is a horizontal pitch of a screen, Pho is a horizontal pitch at the central portion of the shadow mask 70, and Phd is a horizontal pitch at the circumference of the shadow mask 70.
As depicted in FIG. 4, in a horizontal pitch Ph of the conventional color CRT screen, the slots 72 are arranged at regular intervals from the central portion toward a long axis, a short axis and a diagonal axis about X, Y axes of the shadow mask 70 or the slot interval is increased or decreased according to a certain X, Y functions or a slot column function.
And, the horizontal pitch Ph of the screen is set according to a definition of an expected screen of the color CRT. In general, the horizontal pitch Ph set as 0.095%xcx9c0.115% of the screen long side length (W) at the central portion, and a slot horizontal width (Sw) at the central portion is 27%xcx9c28% of the horizontal pitch Pho at the central portion of the mask 70.
The shadow mask 70 passes a curved surface fabrication process, is welded to the frame 30 and fixed to the internal surface of the panel 10 so as to have a certain distance from the interior of the panel 10 in order to disperse and land the red, green, blue electron beam 60 emitted from the electron gun 130 on the screen of the panel 10 at which the fluorescent surface 40 is formed.
In the above-described shadow mask 70, a resolution meaning a capacity for reproducing a picture is described as a product of a shadow mask definition and an electric beam definition, the definition varies depending on the shadow mask definition rather than the electron beam definition.
The shadow mask definition can be described as Equation 1.                                           Mask            -                          MTF              max                                =                                    cos              ⁡                              (                                                      θ                    2                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      P                    h                                    ⁢                  μ                                )                                      =                          2              ⁢                              {                                                      1                                          2                      ⁢                                              P                        h                                            ⁢                      μ                                                        -                                      (                                                                  1                                                  P                          h                                                                    +                      0.5                                        )                                                  }                                                    ⁢                  
                ⁢                              θ            =                          2              ⁢                              {                                                      1                                          2                      ⁢                                              P                        h                                            ⁢                      μ                                                        -                                      (                                                                  1                                                  P                          h                                                                    +                      0.5                                        )                                                  }                                              ,                      μ            =                          N                              2                ⁢                W                                                                        [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
*Mask-MTF: definition of a shadow mask
Ph: horizontal pitch of a shadow mask
*m: spatial frequency
*N: the number of definition pixels
*W: screen long side length
When a definition (Mask-MTF) value of the shadow mask 70 is not less than 0.5, the horizontal pitch (Pho) applied to the central portion of the shadow mask 70 has a pertinent definition reproduction capacity. Herein, the 0.5 definition value was set on the basis of a bright ratio of a picture reproduced for a person having ordinary eyesight.
In addition, in consideration of an eyesight angle of a viewer, when a screen is divided into three equal parts, not right and left parts but the center part is important in perfect reproduction of a high definition picture.
However, in the conventional shadow mask, in particular, a shadow mask in Japan Patent 1999-007901 has a horizontal pitch of 0.088%xcx9c0.123%. As depicted in FIG. 5 and Table 1, it satisfies only low definition and intermediate definition such as a NTSC or a PAL, etc.
When the conventional level horizontal pitch is used for high definition picture reproduction, because it is insufficient to reproduce a high definition picture provided from a broadcast station or a picture equipment, a definition reproduction capacity is low, accordingly a viewer""s sensitivity quality can not be secured.
In addition, when the horizontal pitch is reduced over than a necessity, as depicted in FIG. 6, a purity margin is reduced, it causes problems in color reproduction of a high definition picture.
In general, a purity margin means a possibility in which an electron beam hits a fluorescent portion abutting on a target fluorescent portion without landing on the pertinent fluorescent portion. FIG. 6 describes it.
As depicted in FIG. 6, there are a screen horizontal pitch (Ph), a screen fluorescent band (a), a screen black matrix band (d) and an electron beam horizontal width (b) as main factors determining a purity margin (c).
The screen horizontal pitch (Ph) is formed on a screen by multiplying a horizontal pitch (Pho) on the central portion of the shadow mask 70 by a pitch magnifying power (xcex1) determined by a distance between a screen and a shadow mask and an interval ratio between the screen and a panel, the horizontal pitch (Pho) at the central portion of the shadow mask is considered importantly in determining a screen horizontal pitch (Ph).
And, an electron beam horizontal width (b) is obtained by multiplying a slot width (Sw) of the shadow mask 70 by an electron beam magnifying power (xcex2).
And, the screen fluorescent band (a) and the block matrix band (d) are determined by considering a brightness, etc.
As an example of FIG. 6, in a color cathode ray tube including a 32 inches shadow mask having a width of 0.25 mm and a long side effective surface length of 662.4 mm, a horizontal pitch (Pho) at the central portion of the shadow mask as 0.095% of the screen long side length (W) is 0.629 mm, and a magnified screen horizontal pitch (Ph) is 0.660 mm.
When the fluorescent band (a) is 0.120 mm, the black matrix band (d) is 0.100 mm, a shadow mask slot width as 27% of the shadow mask horizontal pitch is 0.170 mm, when the electron beam magnifying power (xcex2) is applied to the shadow mask slot width, an electron beam horizontal width (b) is 0.229 mm.
Herein, a purity margin in one direction is 0.045 mm.
However, when a front surface of the screen is rotated toward the four directions in consideration of a picture watching direction, an electron beam in which a motion direction is varied by an earth magnetic field may hit a screen fluorescent portion abutting on a target screen fluorescent portion, herein it is important to maintain a color purity despite of that variation, it is called a direction rotation margin.
In order to satisfy the direction rotation margin characteristic, a margin of 0.015 mm is required, however there is a margin of 0.010 mm in consideration of a margin of 0.020 mm on the basis of error occurrence in the fabrication process, the margin is two little, accordingly a yielding rate is lowered.
In more detail, the less the fluorescent band on the central portion, the more the purity margin is secured. However, when the screen fluorescent band (a) is smaller than 0.100 mm, possibility of screen picture spreading is increased due to the screen fluorescent formation error, a luminance (brightness) and a luminance uniformity is lowered due to reduction of electron beam utilization ratio as a ratio of the screen fluorescent band (a) and the electron beam horizontal width (b) calculated as Equation 2, the screen fluorescent band (a) has not to be smaller than 0.100 mm.
luminance=luminescent calibrating constantxc3x97mask transmissibilityxc3x97glass transmissibilityxc3x97electron beam utilization ratioxe2x80x83xe2x80x83[Equation 2]
Accordingly, when a pitch (Pho) at the central portion of the shadow mask determining a screen horizontal pitch (Ph) is set small, a purity margin can not be secured and a color purity may be lowered, accordingly it may deteriorate a color reproduction.
Accordingly, it is required to set an appropriate shadow mask slot horizontal width (Sw) which is capable of solving a purity margin decrease problem according to a horizontal pitch (Pho) setting at the central portion of the shadow mask obtainable a definition and a horizontal pitch reduction at the central portion of the shadow mask.
In order to solve the above-mentioned problem, it is an object of the present invention to provide a shadow mask for a color cathode ray tube which is capable of minimizing definition lowering occurred in a screen according to a screen horizontal pitch increase and preventing color purity lowering according to a screen horizontal pitch decrease.
In order to achieve the above-mentioned object, in a shadow mask for a color cathode ray tube including a plurality of slots in which electron beams pass, a roughly rectangular-shaped effective surface having long sides and short sides and a skirt portion downwardly curved-extended from the outer surface of the effective surface, wherein a horizontal pitch at the central portion of a shadow mask satisfies a condition of Wxc3x970.08%xe2x89xa6Phoxe2x89xa6Wxc3x970.086% when a horizontal pitch at the central portion of a shadow mask slot is Pho, a screen effective surface long side length is W and the shadow mask central portion is placed on the center in which the shadow mask effective surface long side is divided into three equal parts.
In addition, in the present invention, a shadow mask satisfies a condition of Phoxc3x970.24xe2x89xa6Swxe2x89xa6Phoxc3x970.30 when a slot horizontal width at the shadow mask central portion is Sw.